Field of the Invention
The present invention relates to a protection device, especially to an under voltage protection device.
Description of Prior Art
Line voltage may reduce or even become zero due to short fault and these results in damage to circuit or electric equipment. For example, the electric machine may have excessive current (multiple of rated current) caused by the breakdown thereof. The electric machine may suffer to excessively low voltage due to self-starting of the electric machine after electric voltage resumes to supply. The electric machine will fail when line voltage is reduced to a threshold voltage and under voltage protection mechanism is activated to protect the electric machine. When line voltage is reduced below the threshold voltage, no-voltage protection mechanism is activated to protect the electric machine from self-starting.
Taiwan patent No. 1354423 (also published as US Patent Application No: 2007/0014159) discloses an under voltage protection device. As shown in FIG. 2 of this patent, the under voltage protection device comprises a starting unit and a voltage detector with a resistor R3, a Zener diode D4, a comparator U1 and a voltage divider (R4, R5). The starting unit has two first diodes D1, a plurality of resistors R1, R2, a first switch Q1, a second switch Q2, a second diode D2, a Zener diode D3 and a capacitor C1.
In above-mentioned under voltage protection device, the voltage detector first receives an input voltage divided by the voltage divider (R4, R5) and then the comparator U1 compares the divided input voltage with a reference voltage. When the divided input voltage is higher than the reference voltage, the comparator U1 sends a positive output voltage to turn on the second switch Q2 and then turn on the first switch Q1 to activate the fan. On the contrary, when the divided input voltage is smaller than the reference voltage, the comparator U1 sends a negative output voltage to turn off the second switch Q2 and then turn off the first switch Q1 to stop the fan, thus providing under voltage protection.
However, the above mentioned under voltage protection device uses considerable amount of electronic component and is difficult to manufacture. Moreover, the voltage divider (R4, R5) uses resistors R4, R5 of fixed resistance and the under voltage protection device cannot be adapted to input voltages of different amounts. The application of the above mentioned under voltage protection device is limited.